New MM5 have different main language for plugins (JavaScript for MM5 vs VBS for MM4), but you can use COM object for both (but very limited in MM5). For MM5 you can use Chrome Devtools API to call JavaScript from C# (you can check other discussion here).

I will more than likely need to bridge both MM4 and 5 as I cannot see everyone upgrading and it could take a good while for people to make that move.

Even though I'm testing MM5, personally I'm much more comfortable with the MM4 layout and skin. It will take people a while to get used to it or for alternate skins to appear. So for the plugin to be successful it will need to span both versions.

As for the plugin, I'm a DJ and the software I use has it's own database.

I'm looking to find a way to keep the two in step.

The software is great for DJing, but not for music management.

Putting a polished management front end in place could be very interesting...

Other (not currently practicing) DJ here, what do you use? My workflow in previous years was to import my tracks into media monkey, sort into their respective folder structure (music/<album artist>/<album>/<track#> - <track name> ... this coincidentally is what Plex likes to play nice with), feed those tracks into MixedInKey for camelot wheel and BPM tagging, then import those files into rekordbox. It would be great to keep some sets of playlists in sync, ie my genre-specific playlists would be great to have in sync between MM and rekordbox, instead of having to sync one way form MM to rekordbox, or to keep play counts from rekordbox updated in MM as well. Would love to hear your thoughts!

Hey Corneloues,
check out MediaMonkeyNet, a C# wrapper library around the MediaMonkey chromium API which also includes a small sample project. Rainmeter-Mediamonkey is another sample project that implements the library to bridge MM to rainmeter.

Generally speaking, what you want to do is hook into the chromium remote API, from there you can execute generic javascript statements to either do something in MM or retrieve data. Except for a couple for some low-level functionality, most of the MM frontend is running on javascript, so everything is transparent and should be, in theory, extendable.

That said, some of your requirements sound pretty involved, so I'm pretty sure it won't just be a weekend project.